Modern logistics was to a great degree enabled by the development of the product code system and bar codes. Products and pallets could be quickly identified by reading a bar code with an optical reader, and the tracking of items in the logistic chain became feasible. Furthermore, the products could be counted and sold at the point of sale more quickly and more reliably. Beyond consumer product logistics, industrial material management systems, postal and courier services, healthcare and bio-analytics systems and many other fields of human activity have benefited from the use of bar codes.
Over time, the needs of having various kinds of information available on a product outgrew the capabilities of a simple bar code. To this end, new technologies were developed, such as two-dimensional bar codes. Of such technologies, radio frequency identification (RFID) has rather quickly become the technology of choice for identifying and tracking items. RFID technology has a vast number of applications making use of the ability to read an RFID tag from a distance even without a line-of-sight connection between the reader and the tag. RFID is quickly replacing or at least augmenting the bar code technology in many places.
The basic information that an RFID tag carried has traditionally been electronic product code (EPC) information and/or tag identification information. RFID tags with more capabilities may have the ability to store more information in the tag memory, and even carry out some simpler processing of data.
A radio frequency identification tag (RFID tag) typically comprises an RFID chip and an antenna connected to the chip, attached to or protected inside a plastic layer. The RFID chip contains analog and/or digital processing circuitry, e.g. a state machine and/or a processor and memory for storing information, as well as circuitry for receiving radio frequency (RF) energy and modulating the RF wave. The chip is connected to the antenna, whose shape and size depend on the frequency at which the tag operates and the application for which the tag is used. The chip and the antenna are often laid inside a thin plastic container for protection. The tag as a whole is typically flexible to a certain degree, especially the passive tags that do not contain a power supply.
Passive tags use the energy from the radio-frequency electromagnetic field of the read-out signal to power the operations that the tag carries out. Passive tags operate essentially while they are in the reader field, and are essentially inactive at other times. Therefore, the farther the reader device and the weaker the read-out signal, the less energy is available for the tag to use. This in turn means that the tag cannot perform very complex operations that would require a lot of power, since such tags could only be read from a very short distance away. In other words, the read-out distance of a tag is to a large extent determined by the power consumption requirements of the tag. Naturally, the attenuation and power distribution of the electromagnetic signal emitted by the tag is another concern. To tackle this issue, some passive tags have a battery that may be used to power the processing circuitry and thereby allow longer operating range for the tag. Such tags that use an internal power source to energize the response signal may be called semi-passive tags.
Some applications may necessitate more complex processing and/or more demanding properties of the RFID tag. It may become necessary to arrange more internal power such that these operations may be carried out. Some tags may require an internal power source to communicate with a longer range. Such tags may be called active tags. It may also be necessary to provide internal power so that operations can be carried out at times when the power from the external radio frequency electromagnetic field is not available. Usually this is arranged by means of a battery attached to the RFID tag. The battery may power sensors, memory, processors and/or a transmitter of the tag so that sensing, data logging and processing may be carried out at any time. Such more complex tags may be more complicated to manufacture, more expensive and/or more prone to malfunction than passive or semi-passive tags. Moreover, any tags with an internal power supply may have a limited lifetime, since they cannot be operated properly or at all when the battery runs out. Another practical problem is that when an active tag is taken into use, the battery operation needs to be activated. Otherwise, the battery would be in use e.g. already starting from manufacturing of the tag and prior to actual active use.
An example of demanding properties of a tag are measurements. In some applications, wireless measurements regarding the environment may help process or quality control. Active tags are often larger and more expensive than passive tags. Furthermore, in some applications, particularly related to construction engineering, the battery of the active tag may be difficult to change. Therefore, e.g. for process or quality control, passive tags capable of measurements are needed.